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The notable changes:
. Do not allow horizontal dragging of the split windows.
. Do not show the 'Split' button when dragging a non-split window. Instead,
  highlight the split windows as drop targets.
. When a window is dropped on top of a left/right split window, switch to that
  window in that part of the split.

BUG=404119
R=oshima@chromium.org

Review URL: https://codereview.chromium.org/537073002

Cr-Commit-Position: refs/heads/master@{#293202}